The present invention relates to methods and apparatus for cutting expanded graphite sheet.
Electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes both comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The fuel fluid stream which is supplied to the anode may be a gas such as, for example, substantially pure hydrogen or a reformate stream comprising hydrogen. Alternatively, a liquid fuel stream such as, for example, aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
Solid polymer fuel cells employ a solid polymer electrolyte, otherwise referred to as an ion exchange membrane. The membrane is typically interposed between two electrode layers, forming a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). While the membrane is typically selectively proton conductive, it also acts as a barrier, isolating the fuel and oxidant streams from each other on opposite sides of the MEA. The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, and to ensure adequate sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
Fuel cell plates known as fluid flow field plates have open-faced channels formed in one or both opposing major surfaces for directing reactants and/or coolant fluids to specific portions of such major surfaces. The open-faced channels also provide passages for the removal of reaction products, depleted reactant streams, and/or heated coolant streams. For an illustration of a fluid flow field plate, see, for example, U.S. Pat. No. 4,988,583, issued Jan. 29, 1991. Where the major surface of a fluid flow field plate faces an MEA, the open-faced channels typically direct a reactant across substantially all of the electrochemically active area of the adjacent MEA. Where the major surface of a fluid flow field plate faces another fluid flow field plate, the channels formed by their cooperating surfaces may be used for carrying coolant for controlling the temperature of the fuel cell.
Fluid flow field plates may also have apertures therein. For example, fluid flow field plates may have manifold openings for supplying and exhausting reactants and/or coolant to and from the channels. When assembled in a fuel cell stack, such manifold openings in the plates of adjacent fuel cells cooperate to form the manifolds for supplying and exhausting reactants and/or coolant to and from the stack.
Conventional methods of fabricating fluid flow field plates require the engraving or milling of flow channels (and optionally apertures) into the surface of rigid plates formed of graphitized carbon-resin composites. These methods of fabrication place significant restrictions on the minimum achievable cell thickness due to the machining process, plate permeability, and required mechanical properties. Further, such plates are expensive, both in raw material costs and in machining costs. The machining of channels and the like into the graphite plate surfaces causes significant tool wear and requires significant processing times.
Alternatively, fluid flow field plates can be made by a lamination process, as described in U.S. Pat. No. 5,300,370, issued Apr. 5, 1994, in which an electrically conductive, fluid impermeable separator layer and an electrically conductive stencil layer are consolidated to form at least one open-faced channel. Such laminated fluid flow field assemblies tend to have higher manufacturing costs than single-layer plates, due to the number of manufacturing steps associated with forming and consolidating the separate layers.
Alternatively, fluid flow field plates can be made from an electrically conductive, substantially fluid impermeable material that is sufficiently compressible or moldable so as to permit embossing. Expanded graphite sheet is generally suitable for this purpose because it is relatively impervious to typical fuel cell reactants and coolants and thus is capable of fluidly isolating the fuel, oxidant, and coolant fluid streams from each other; it is also compressible and embossing processes may be used to form channels in one or both major surfaces. For example, U.S. Pat. No. 5,527,363, issued Jun. 18, 1996, describes fluid flow field plates comprising a metal foil or sheet interposed between two expanded graphite sheets having flow channels embossed on a major surface thereof.
However, forming apertures in expanded graphite sheet can be problematic, especially in the context of high-volume manufacturing. Expanded graphite sheet is formed from expanded graphite particles compressed into thin sheets. The sheet comprises aligned expanded graphite particles and constituent layers of carbon atoms parallel to the surface of the sheet. A conventional approach to making apertures in the sheets involves punch cutting the apertures, in which mating male and female punch features shear or tear the expanded graphite sheet. Punch cutting may be performed during embossing or in a separate step following embossing. Due to the nature of the material, however, punch cutting typically results in apertures with poor or inconsistent edge quality, material flaking around the edge of the aperture, and material cutout failures. In addition, where punch cutting occurs in a separate step following embossing, inconsistently placed apertures can also be a problem.
It would be desirable to have a method and apparatus for forming apertures in expanded graphite sheet flow field plates having improved edge characteristics, in an efficient and repeatable manner.
In one embodiment, the present method for cutting an expanded graphite sheet comprises:
(a) urging at least one cutting tool against the sheet, the cutting tool having at least one ridge extending therefrom, the ridge comprising a substantially tapered cross-section having sloping sides and an edge surface;
(b) displacing at least a portion of the material comprising the sheet as the ridge is urged against the sheet; and
(c) compressing a region of the sheet in contact with the at least one ridge so that the density of the region at least reaches the breaking density of the sheet.
The expanded graphite sheet may comprise at least one embossed fluid flow field plate. In another embodiment, the present method may further comprise embossing the expanded graphite sheet to form at least one fluid flow field plate.
The ridge(s) may define the perimeter of an aperture. The ridge(s) may comprise a plurality of ridges, each of the ridges defining the perimeter of an aperture.
In the present method, the density of the compressed region of the sheet may reach at least 2.2 g/cm3.
In the present method, the cutting tool(s) may be a die, such as a reciprocal die or a roller die, for example.
In a further embodiment of the present method, step (a) may further comprise urging a pair of cutting tools against opposing major surfaces of the sheet, wherein each of the cutting tools has at least one ridge extending therefrom, with the ridges opposing one another. Step (c) may further comprise compressing a region of the sheet interposed between the opposing ridges so that the density of the region at least reaches the breaking density of the sheet.
Where the cutting tool is a die, it may have embossing features incorporated therein. Where opposing cutting tools are employed in the present method, each of the dies may comprise a plurality of ridges, and each pair of opposing ridges may define the perimeter of an aperture. The opposing ridges may approach each other without making contact as the pair of cutting tools are urged towards each other. Again, the dies may be reciprocal dies or roller dies, if desired.
One embodiment of the present apparatus for cutting an expanded graphite sheet comprises:
(a) at least one die adapted to be urged against the sheet, the die(s) having at least one cutting ridge extending therefrom, the ridge(s) comprising a substantially tapered cross-section having sloping sides and an edge surface; and
(b) at least one compression mechanism adapted to urge the die(s) against the sheet.
In another embodiment, the present apparatus further comprises a pair of dies adapted to receive the sheet therebetween, each of the dies having at least one cutting ridge extending therefrom, wherein the ridges on respective dies oppose one another, and the compression mechanism is adapted to urge the dies against the sheet.
In the present apparatus, a ridge on a die may define the perimeter of an aperture. For example, where the present apparatus comprises opposing dies, each of the dies may comprise a plurality of cutting ridges, wherein each pair of opposing ridges defines the perimeter of an aperture.
In the present apparatus, at least one die may have embossing features incorporated therein for forming at least one fluid flow field plate from the sheet.
Where the dies comprise reciprocal dies, the compression mechanism may comprise a press platen. Where the present apparatus comprises a pair of opposing roller dies, the roller dies are capable of forming a nip therebetween that is less than the thickness of the sheet, and the compression mechanism may comprise feeding the sheet between the roller dies.
In the present apparatus, the width of the edge surface of a ridge may be less than or equal to about 300 xcexcm. The sloping sides of a ridge may be chamfered or radiused.